Chemistry Reference
In-Depth Information
chloride) can be made by adding dioctyl phthalate to a blend of polystyrene and
rigid PVC.
The most generally useful compatibilizing agents are copolymers in which
each different monomer or segment adheres better to one or other of the blend
ingredients. Applications of copolymers are classified here according to structure
as statistical, block, or graft copolymers. This seems to be as useful a framework
as any within which to organize the review, but it has no fundamental bearing on
the properties of blends, and different copolymer types may very well be used in
similar applications in polymer mixtures.
It is interesting that a mixture of poly-A and poly-B can sometimes be stabilized
by addition of a copolymer of C and D, where A, B, C, and D are different mono-
mers. This occurs if the intermolecular repulsion of C and D units is strong enough
that each of these monomer residues is more compatible with one or other of the
homopolymer ingredients than with the comonomer to which it is linked chemically.
Statistical Copolymers. The term statistical is used here to refer to copolymers
in which the sequence distribution of comonomers can be inferred statistically
from the simple copolymer model (Chapter 9) or alternative theory. In the present
context “statistical copolymers” excludes block and graft structures and incorpo-
rates all other copolymers. It is useful first of all in this section to point out that
statistical copolymers are not mutually miscible if the mixture involves abrupt
changes in copolymer composition. Coatings chemists observe this phase separa-
tion as haze (internal reflections) in films.
Note also that although a conventional high conversion vinyl copolymer may
exhibit a wide range of compositions (depending on the reactivity ratios of the
comonomers and the monomer feed composition), there are generally so many
mutually miscible intermediate compositions that the extremes can be expected to
blend well with the rest of the mixture.
Use of statistical copolymers in blends is usually predicated on the existence
of a specific interaction between one of the comonomers in the copolymer and
other ingredients in the mixture. Thus PVC is miscible with the ethylene/ethyl
acrylate/carbon monoxide copolymers
[31]
. The homogenizing effect here is a
weak acid
base interaction between the carbonyl of the copolymer and the
weakly acidic hydrogen atoms attached to the chlorine carrying carbons of the
PVC. Ethylene/vinyl acetate/carbon monoxide copolymers are more miscible with
PVC, and ethylene/vinyl acetate/sulfur dioxide copolymers are miscible with the
same polymer over a very wide composition range.
The morphology and stability of mixtures of PVC with copolymers depend on
the composition and mixing history of the blend as well as on the nature of the
copolymer. Ethylene/vinyl acetate copolymer is reported to behave essentially as
a lubricant between PVC particles at low copolymer concentrations and to begin
to form single-phase compositions with PVC with increasing copolymer content
in the blend. This situation changes with increasing vinyl acetate content in the
copolymer and increasing mixing temperatures, both of which increase the solu-
bility of the copolymer in PVC.
